Hi! *WELCOME*! I 'empathize' with your goal/interests. (I'm a semi-Newbie & Unix OldTimer)
I had some great success with NuTyX (in VirtualBox [on my poor ol' XP netbook & slow inet])
I stripped /boot way down to just a few files so that I fully understood...
And then put /boot on a separate drive [.vdi] that isn't even mounted, lol.
(my grub.cfg is just a few simple/trivial lines & i386pc has only a dozen files!)
All this helped a lot in learning the depths of grub. I have only about 70 'pkgs', so I can keep re-installing [experimenting] ultra fast, esp. cuz it doesn't rebuild initrd. (&no GUI)
initrd is the next thing I might study/play-with in-depth. Then maybe build a custom kernel.
has a recent 4.4 kernel (/proc/config.gz has the kernel options)
I'd suggest a bit of skim-reading LFS, and lots&LOTS of crafty web searches ;-)
Best wishes! Enjoy! (p.s. notice "Similar Threads" at bottom of this page! Cool LQ feature!)